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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"



Reagents, (i) Potassium permanganate solution prepared by dissolving
3-2 grams of the pure salt in boiled distilled water and making up to i litre.

(2) Sodium arsenite solution, obtained by dissolving r6 gram of pure
arsenious anhydride and o-S gram of pure sodium hydroxide in water,
heating if necessary, and making up to i litre : i c.c. of this solution corre-
sponds with about 0-5 c.c. of the permanganate solution.

Titration of the -permanganate solution (Sorensen). About 0-3 gram of
pure sodium oxalate, in minute crystals and dried at 100, is weighed exactly,
dissolved in 500-600 c.c. of boiling water, mixed with 50 c.c. of dilute sul-
phuric acid (i vol. acid to 5 vols. water) and the permanganate solution
run in from a burette until a faint pink colour persists. Since 670 grams
of sodium oxalate are equivalent, as regards permanganate, to 164-79 grams
of manganese as salt, the amount of manganese, x, corresponding with the
permanganate used is given by

670-0 :164-79 '' P ' x>

where P is the quantity of the oxalate taken. The quotient of x by the
number of c.c. of permanganate used gives the amount of manganese corre-
sponding with i c.c. of permanganate.1

Procedure. 1-2 grams of ferro-manganese or 2-5 grams of spiegeleisen
are treated with nitric acid (D = 1-18) according to the conditions described
under 2 (p. 172). The solution is evaporated, ignited to decompose the
nitrates, taken up in hydrochloric acid and the silica rendered insoluble,
filtered and, if required, weighed. The hydrochloric acid solution, con-
taining the manganese, when cold is made up to 250 or 500 c.c. in a measur-
ing flask. The titration of the manganese is carried out on aliquot parts
of the solution, each containing 0-04-0-08 gram of manganese.

If the sample is of high manganese content and hence contains too
little iron, it is well to add to each portion 510 c.c. of ferric chloride solution
(500 grams of pure ferric chloride dissolved in water acidified with hydro-
chloric acid and the volume made up to i litre). In this case it is necessary
to ascertain, by a blank test under similar conditions, whether the ferric
chloride absorbs permanganate and, if so, to allow for this in the calculation.

Preliminary test. Before titrating, a trial test must be made to establish
the quantity of permanganate to be added.

An aliquot part of the solution is treated in a litre flask with a few drops
of 30% hydrogen peroxide to oxidise any trace of ferrous salt and then
heated to boiling to expel the excess of the oxidising agent. After 1015
minutes' boiling, the volume is made up to 600-700 c.c. with boiling water,
a suspension of zinc oxide 2 in water being then added in small amounts
and with shaking until all the iron is precipitated in brown flocks (not pale

1  The reaction between sodium oxalate and permanganate takes   place  thus :
5Na2C204 + 2KMn04 + 8H2SO4 = 2MnSO4 + K2S04 + 5NaaSO4 + ioCO2 + 8H2O,
and that between a manganese salt and permanganate in the "sense,

3MnCl2 + 2KMn04 + 2H2O = 2KC1 + 5Mn02 + 4HC1.

Hence 2 mols. of permanganate correspond with 5 of oxalate and with 3 of man-
ganese salt, so that 5 mols. of oxalate (670-0) = 3Mn (164-79).

2  Namias suggests that the zinc oxide be ground with sodium hypochlorite solution,
left for some days and then washed several times by decantation (Ind. Chim. Min. e
Metall., 1915, II, p. 397).m of sulphur,